Actuators have long been used in heating, ventilation and air conditioning (“HVAC”) system systems to control the movement or location of a flow blockage member in a damper or valve. While actuators can use pneumatic or other sources of power, most HVAC actuators include an electrically powered motor to turn a rotary output. The motor shaft is typically significantly geared down, so numerous turns of the motor shaft turn the rotary output a much smaller amount such as 90°. The rotary output is connected, typically in the field, to a drive stem or drive shaft of a damper or valve as a component of the building's HVAC system. U.S. Pat. Nos. 8,789,807 and 9,276,449, and U.S. patent application Ser. No. 14/939,577, all owned by the assignee of the present invention, are all incorporated by reference for their general teachings regarding HVAC actuators.
Some valve and damper components of HVAC and other systems for controlling fluid flow through a duct or pipe include two flow blockage members, such as two butterfly valve blockage plates. U.S. Pat. Nos. 3,070,345, 3,633,626, 3,633,626, 3,934,851, 5,168,900, 5,647,399, 5,794,647, 6,484,755, 6,568,417, 7,140,392, 7,267,139, 7,658,671, 8,342,206, 8,398,468 and 9,404,594 show such arrangements, and are all incorporated by reference for examples of environments where the present invention may be employed. In many instances, both of the flow blockage members in such dual arrangements are mounted for coaxial, rotational movement. In some instances, the control system is required to actuate the two flow blockage members entirely independently, i.e., where each of the flow blockage members can be selected and positioned at whatever percentage open or closed is desired, regardless of the position of the other flow blockage member, and without regard to which flow blockage member is opened first. In other instances, one of the two flow blockage members is always the first to begin to open (during opening from a fully closed position), while the other flow blockage member is always the first to begin to close (during closing from a fully open position).
As can be seen by a review of the patents listed above, there are many actuator structures which can be used on dual coaxially arranged flow blockage valves and dampers. Each of these prior art actuator structures has its own drawbacks, often in terms of cost or complexity, and in terms of difficulty of use and control. A simple, robust, low cost actuator design which can control dual flow blockage valves and dampers is needed.